Makes 6-8 servings. Recipe from Alton Brown.
Process the onion in a food processor for 10 to 15 seconds and turn out into the center of a tea towel. Gather up the ends of the towel and squeeze until almost all of the juice is removed. Discard juice.
Return the onion to the food processor and add the lamb, garlic, marjoram, rosemary, salt, and pepper and process until it is a fine paste, approximately 1 minute. Stop the processor as needed to scrape down sides of bowl.
For Tzatziki sauce, place the yogurt in a tea towel, gather up the edges, suspend over a bowl, and drain for 2 hours in the refrigerator (or just dump it in the bowl). Place the chopped cucumber in a tea towel and squeeze to remove the liquid; discard liquid. In a medium mixing bowl, combine the drained yogurt, cucumber, salt, garlic, olive oil, vinegar, and mint. Serve as a sauce for gyros. Store in the refrigerator in an airtight container for up to a week.
Preheat the oven to 325 degrees F.
Place the lamb mixture into a loaf pan, making sure to press into the sides of the pan. Place the loaf pan into a water bath and bake for 60 to 75 minutes or until the mixture reaches 165 to 170 degrees F. Remove from the oven and drain off any fat. Place the loaf pan on a cooling rack and place a brick wrapped in aluminum foil directly on the surface of the meat and allow to sit for 15 to 20 minutes, until the internal temperature reaches 175 degrees F. Slice and serve on pita bread with tzatziki sauce, chopped onion, tomatoes and feta cheese.